1. Field of the Invention
The present invention relates to a magnetic recording medium, and more particularly to a magnetic recording medium having excellent surface smoothness and superior electromagnetic conversion characteristics.
2. Disclosure of the Related Art
A conventional magnetic recording medium comprises a magnetic layer on one side of a non-magnetic support and a back coat layer on the other side of the non-magnetic support to enhance the running durability.
In recent years, increases in the quantity of data being recorded have resulted in demands for similar increases in the recording density of magnetic recording media. In order to enable increases in recording density, a recording wavelength continues to shorten, and the thickness of the magnetic layer continues to reduce.
As the thickness of the magnetic layer is reduced, any surface roughness of the support is reflected in the surface of the magnetic layer, causing a loss in the smoothness of the magnetic layer surface and a deterioration in the electromagnetic conversion characteristics. As a result, a non-magnetic layer is provided on the support surface, for example, as an undercoat layer, and the magnetic layer is then provided on this non-magnetic layer.
As increasingly shorter recording wavelengths are being used, it is desirable to provide a flatter and improved mirror surface to the magnetic layer surface in view of spacing loss.
In view of the method for producing a magnetic recording medium, the magnetic layer is subjected to calendering to improve the smoothness of the magnetic layer surface. Calendering of the magnetic layer following formation of the back coat layer on the other side of the support has previously been reported.
For example, paragraph [0022] of Japanese Patent Laid-Open Publication No. Hei 9-185822(1997) reports that by conducting calendering with the magnetic layer and the back coat layer already provided on the support, the surface workability of the magnetic layer improves markedly, resulting in improvements in other characteristics such as the electromagnetic conversion characteristics.
Japanese Patent Laid-Open Publication No. Hei 11-31322(1999) discloses a production process for a magnetic recording medium comprising a magnetic layer on one side of a support, which already comprises a thin film formed by vacuum deposition on at least this side, and a back coat layer provided on the other side of the support, wherein calendering of the magnetic layer is conducted following formation of the back coat layer.
Japanese Patent Laid-Open Publication No. Sho 62-125537(1987) discloses a process in which calendering of the magnetic layer and the back coat layer are conducted simultaneously while the applied coatings are cured by irradiation. However, a non-magnetic layer is mentioned in this publication, and the thickness of the magnetic layer, at 3.0 to 4.8 μm, is considerably high.